1. Field of the Invention
This invention relates generally to analog-to-digital converters and, in particular, to such converters that utilize bi-directional integration to form a charge balance basis for voltage-to-time conversion.
2. Description of the Related Art
Various methods and systems have been devised to perform analog-to-digital conversion. There are feedback methods such as successive approximation and up-down counter tracking that use a digital-to-analog converter whose analog output is compared to the unknown analog signal and the digital input adjusted for minimum difference. When this is achieved the digital value represents the value of the analog input. These converters have short conversion times but quantization is unequal and cost increases rapidly with higher accuracy. There are direct conversion types such as flash or parallel converters that implement the analog-to-digital conversion by using a large number of comparators to achieve very high speed, however with the penalty of very high cost. There are Delta-Sigma types that use a 1-bit DAC in a high-speed charge balance loop whose quantized output is followed by an elaborate low pass digital filter to reduce the quantizing noise.
There are integration type analog-to-digital converters capable of high performance at low cost that operate at slower conversion rates suitable for use in digital multimeters, digital panel meters and other measurement instruments. These converters usually take advantage of their ability to integrate the input signal over one or more exact periods of the power line frequency to gain immunity to power line noise that is present with the input signal. Examples of integration-type converters are single slope, dual slope and multi-slope methods which are based on a voltage-to-time (V/T) conversion where the unknown input voltage is converted to a time period that is measured by counting a clock frequency. Another type is the voltage-to- frequency (V/F) converter that uses quantized charge feedback to balance the charge from the input signal in the integrator and create a frequency proportional to the input value. This frequency is then counted using a fixed timebase.
Ideal performance for the integration type converter could be achieved if it could continuously integrate the input signal over complete periods of the line frequency for power line noise rejection, convert each of these periods at the line frequency rate for high conversion speed and be capable of high resolution and accuracy at low cost. The present V/T converters such as dual slope and multi-slope achieve good linearity and high resolution but are unable to convert at the line frequency rate because they first integrate the signal over one (or more) line frequency periods and then spend the next period or two deintegrating a reference signal and auto-zeroing the circuit. Conversely, V/F converters are capable of converting at the line frequency rate by using consecutive time bases equal to the line frequency period but are unable to generate the high frequencies required for better than 0.01% resolution, while maintaining 0.01% linearity with low component cost.